In the existing cellular system, an antenna array in a base station is generally arranged horizontally. As shown in FIG. 1, in which dual-polarized antennas which are arranged horizontally are illustrated, and FIG. 2 in which an array of antennas which are arranged horizontally is illustrated. Since the array of antennas in the base station is arranged horizontally so that a beam of a transmitter in the base station has a fixed downward inclination angle, and can only be adjusted in the horizontal direction, which result in various existing beam-forming/pre-coding technologies, etc., merely based on channel information in the horizontal direction. In fact, since a radio signal is propagated in three dimensions in a space, the performance of the system cannot be optimized with the fixed downward inclination angle of a beam in the vertical direction, and adjustment of the beam in the vertical direction is of great significance to a reduction in inter-cell interference, and an improvement in performance of the system.
As the antenna technologies are advancing, an array of active antennas in which each array element can be controlled separately has emerged in the industry, thus making it possible to adjust dynamically the beam in the vertical direction. As shown in FIG. 3, in which dual-polarized antennas arranged in the horizontal and vertical directions are illustrated, and in FIG. 4, in which an array of antennas arranged in the horizontal and vertical directions is illustrated. The MIMO technology in the array of antennas arranged in the horizontal and vertical directions will be referred in this context to as FD MIMO or three-dimension (3D) MIMO.
In the 3D antenna array illustrated in FIG. 3 and FIG. 4, a transmitting signal of the base station can be used for beam-forming on a UE in both the horizontal direction and the vertical direction. In order to enable the base station to determine a beam-forming vector in the vertical direction for enabling the beam in the vertical direction to be oriented to the UE so as to maximum a beam-forming gain, the UE typically needs to feed back Channel State Information (CSI) in the vertical direction.
The UE typically feeds back CSI in the vertical direction by transmitting a beamformed CSI measurement pilot. Taking the transmission of Channel State Information Reference Signal (CSI-RS) as an example, the UE is configured with a plurality of CSI-RS resources, and different CSI-RS resources are configured with different vertical beam-forming vectors. The UE feeds back CSI based upon the optimum CSI-RS, and notifies the base station of positional information of the optimum CSI-RS, so that the base station can perform vertical beam-forming using the optimum vertical beam-forming vector. The CSI generally includes a Rank Indication (RI), a Pre-coding Matrix Indicator (PMI), and a Channel Quality Indication (CIQ). In a particular implementation, the UE can report positional information and CSI corresponding to a plurality of optimum CSI-RS resources, so that the base station selects one or more different vertical beams therefrom for transmission of downlink data. In this implementation, the number of ports of a CSI-RS resource is less than the total number of antenna elements, and the same as the number of antenna elements in the horizontal dimension, particularly as illustrated in FIG. 5.
The UE feeds back CSI in the two dimensions, i.e., the vertical and horizontal dimensions, by transmitting a non-precoded CSI-RS, that is, more CSI-RS resource ports are configured, where a part of the CSI-RS resource ports are mapped onto horizontal antenna elements, and the other part of the CSI-RS resource ports are mapped onto vertical antenna elements; and a CSI-RS signal is transmitted in the two dimensions, i.e., the vertical and horizontal dimensions, and the UE can obtain two-dimension CSI according to the CSI-RS to thereby feed back the downlink CSI. In this implementation, more CSI-RS resource ports are required, and the UE can feed back CSI in substantially the same way as the prior art, that is, the UE can measure and make a feedback directly over the CSI-RS resources without feeding back the CSI separately in the respective dimensions.
In a Long Term Evolution (LTE) system, in order to support Cooperative Multiple Point Transmission (CoMP), the concept of a CSI process has been introduced. Each CSI process can correspond to a downlink transmission point, and is configured with a Non-Zero Power (NZP) CSI-RS and an Interference Measurement Resource (IMR). The UE performs channel measurement based upon the NZP CSI-RS, and performs interference measurement over the corresponding IMR to thereby obtain and feed back CSI corresponding to the respective CSI processes. Each LTE UE can be configured with at most three CSI processes to feed back CSI. The UE can feed back CSI over a periodical PUCCH, or an aperiodical PUSCH. When the CSI is fed back periodically, the base station configures a periodical PUCCH resource, and the UE reports corresponding CSI periodically over the configured resource. When the CSI is fed back aperiodically, the base station triggers the UE via DCI to feed back CSI, and the UE feeds back CSI over a PUSCH in an uplink sub-frame corresponding to a triggering sub-frame. For the aperiodical feedback, the base station can trigger the UE to report CSI corresponding to each CSI process in certain set of CSI processes.
At present, only CSI is fed back in a CSI process in the FD MIMO system, so the feedback approach is inflexible, and cannot support the CSI feedback in a scenario in which various CSI related information needs to be fed back after the CSI is measured. For example, the feedback approach cannot support the CSI feedback by transmitting a beamformed CSI measurement pilot (CSI-RS), or that by transmitting a non-precoded CSI-RS.